Use of proportional control valves is widespread in many industries. For example, a proportional control valve may be used to control a position of a machine tool such as a saw requiring accurate positioning and repeatability of that position within a small tolerance. Proportional control valves may also be used in applications where speed control may be required, for example, to maintain a precise speed of a web traveling through a printing press having varying roll sizes. Similarly, proportional control valves may be used in applications requiring force or pressure control such as pressure control for cylinders being utilized to lift or hold a load.
In an effort to optimize proportional control valve performance, manufacturers have attempted (1) to minimize the number of moving and/or contacting components of the valve, (2) to minimize affects of temperature, fluid viscosity, contaminates, etc. on the components of the valve, and (3) to optimize valve operating parameters. Optimizing the valve operating parameters may include increasing the response time of the valve (i.e., the time required for the valve output to reach the new level when the valve input current is stepped in amplitude), improving repeatability (i.e., the valve's ability to return to the same output flow on repeated trials), reducing hysteresis (i.e., the difference in valve input current required to produce the same output as the valve is slowly cycled), and reducing deadband (i.e., the region of no response), to name a few.
One current design used to minimize the number of components in contact with one and another, and to optimize valve operating parameters of a proportional control valve, includes the use of a well-known linear variable differential transform (LVDT). In general, the LVDT is composed of a two secondary coils placed symmetrically on either side of a primary coil contained within a hollow cylindrical shaft, and a moveable solid magnetic core. When coupled to a linearly moveable valve spool, displacement of the magnetic core relative to the hollow cylindrical shaft causes the mutual inductance of each secondary coil to vary relative to the primary coil. As a result, the LVDT provides an electrical output proportional to a position of the magnetic core and therefore the position of the coupled valve spool. Accordingly, subsequent adjustments based on the electrical output can made to the position of the valve spool and a resulting fluid flow through the valve spool. Although effective for providing an electrical output proportional to a position of the valve spool, the LVDT is relatively costly.
Typically, set-up, initializing and overall operational control of a proportional control valve is accomplished using individual electronic components that are remotely located from the valve; they are not resident in the valve housing. Additionally, current designs utilizing such individual electronic components often require manual potentiometer adjustments and manual jumper reconfigurations for set-up and control of the proportional control valve. While the use of individual electronic components simply reflects an inefficient use of space and of currently offered integrated circuit technology, the manual intervention required to maintain, set-up and control typical proportional control valves contributes adversely to the overall cost of operating and maintaining the proportional control valve.